h3 {
    text-align: center;
}

p {
    text-align: center;
    margin: 5px auto;
}

p.title {
    color: blue;
    font-weight: bold;
}

p.desc {
    font-family: 宋体;
}

.wrapper {
    width: 800px;
    border: 1px solid gray;
    margin: 25px auto;
}

.flex-container {
    /* 使被选中的元素成为弹性容器 */
    display: flex;
    border: 1px solid blue;
    margin: 15px;
}

.flex-item:nth-child(2n) {
    background: #ffff00;
}

.flex-item:nth-child(2n+1) {
    background: #dfdfdf;
}

.first {
    flex-direction: row;
    flex-wrap: nowrap;
    height: 100px;
}

.first>.flex-item {
    flex-basis: 150px;
}

.second {
    flex-direction: row;
    flex-wrap: wrap;
    height: 100px;
}

.second>.flex-item {
    flex-basis: 150px;
}

.third {
    flex-direction: row;
    flex-wrap: wrap-reverse;
    height: 100px;
}

.third>.flex-item {
    flex-basis: 150px;
}

.fourth {
    flex-direction: column;
    flex-wrap: nowrap;
    height: 200px;
}

.fourth>.flex-item {
    flex-basis: 40px;
}

.fifth {
    flex-direction: column;
    flex-wrap: wrap;
    height: 200px;
}

.fifth>.flex-item {
    flex-basis: 40px;
}

.sixth {
    flex-direction: column;
    flex-wrap: wrap-reverse;
    height: 200px;
}

.sixth>.flex-item {
    flex-basis: 40px;
}
